Not applicable.
This invention is in the field of surface plasmon resonance sensors.
Surface plasmon resonance (SPR) sensors are known for use in detecting properties of materials. In conventional SPR sensors, light at a selected wavelength is directed through a high refractive index medium to a sample under analysis, which is expected to have a lower refractive index. A resonance film, typically a very thin gold film is disposed between the two media. This film typically reflects the incident light, but electrons of some of the atoms at the medium interface resonate between conduction bands (surface plasmon resonance). In addition, because the resonance film is extremely thin, (e.g., on the order of 500 xc3x85), an electromagnetic field component of the incident light penetrates a very short distance into the surface of the lower refractive index material (the sample medium), even when the light is substantially reflected from the film. This penetration is in the form of an exponentially attenuating evanescent wave. For incident light that is monochromatic and polarized, there is a specific angle of incidence at which the light is absorbed rather than reflected, due to resonance energy transfer between the evanescent wave and the surface plasmons. This angle, at which the reflected light intensity is at a minimum, is influenced by the properties of the material adjacent to the thin gold film. In conventional SPR sensing, a photodiode array detects the intensity of light reflected from the gold film and sample medium over a range of angles. The array is thus able to generate a signal indicating which photodiode is receiving the minimum intensity light, and thus indicating the angle of maximum absorption by the medium.
U.S. Pat. Nos. 6,045,756, 5,898,503, 5,912,456, and 5,946,083 commonly assigned herewith and incorporated by reference into this specification, describe various integrated SPR sensors. These disclosed sensors are integrated, in that the light source, light detector, high refractive index material, and gold film are all encapsulated into a single small package. The miniaturized package provided by such integration is thus well-suited for many applications, including in-line process control sensing. In addition, the integration of the SPR sensors into a monolithic device results in an optically robust device, as the optical path within the sensor remains fixed and constant. Furthermore, this integration of the sensor reduces the cost of manufacture of the sensor, and thus further facilitating widespread use of the technology. In use, the incident angle at which polarized light is absorbed by the sample medium applied to an SPR sensor is a measure of the refractive index of the sample medium. As known in the art, the refractive index of a material is determined by the composition of the material. Accordingly, measurement of the refractive index is now used to determine the purity of a supposedly pure compound, and to determine the composition of a simple mixture, such as a solution of sugar and water.
As such, SPR sensing has applicability in many manufacturing and analysis applications, including chemical processing and analysis, process control, pollution detection, and the like. For example, an important measure in the beverage industry is the sugar concentration in the beverages being produced. This measurement is not only necessary for sweet beverages, such as sweetened carbonated beverages and naturally sweet juices and drinks, but is also necessary for other less-sweet beverage, particularly alcoholic beverages such as beers and wines. SPR sensing of the sugar concentration is therefore an attractive technology in the manufacture and processing beverages.
SPR sensors are now conventionally used in spot, or sample, measurements, as well as for in-line real-time process control. For spot measurements, SPR sensors are used in handheld and tabletop instruments, where the sample to be measured is applied over a window in the instrument at which the resonance film resides; a visual display of the result of the SPR sensing is then provided by the instrument. In-line measurements are made by deploying the SPR sensor into a pipe or nozzle through which the liquid being measured passes.
It has been observed in connection with the present invention, however, that conventional SPR sensors are not sufficiently robust to provide a reasonable reliable operating life. In the case of hand-held instruments, the sample window must be wiped clean after measurement, in preparation for the next sample. However, the thin gold resonance film of conventional SPR sensors cannot withstand this wiping, and therefore conventional SPR sensors are not suitable for use for this operation. In the case of in-pipeline real-time monitoring, abrasive liquids such as tomato paste damage the thin and fragile resonance film over time. Conventional SPR sensors are therefore not useful in many of these applications, as well.
It is therefore an object of the present invention to provide a robust surface plasmon resonance sensor that has a surface plasmon layer that can withstand repeated wiping and cleaning.
It is a further object of the present invention to provide such a sensor that can be readily integrated and miniaturized so as to be useful in process pipe applications, and sufficiently robust to be used to monitor flowing abrasive liquids.
It is a further object of the present invention to provide such a sensor that can be easily manufactured and deployed.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.
The present invention may be implemented by applying a hard thin film on the exposed surface of the surface plasmon film. This hard film must be sufficiently hard and robust so that an extremely thin layer of the hard film can adequately protect the surface plasmon film, while still permitting a reasonable thickness of the sample surface to be within the range of the evanescent resonance wave.